Alzheimer´s disease (AD) is a neurodegenerative disorder defined by progressive memory loss and cognitive impairment. AD is the most common dementia at old age. The definite diagnosis is possible postmortem only. AD is characterized by the presence of two abnormal protein deposits: Amyloid plaques composed of extracellular deposits of β-amyloid (Aβ) peptides, and neurofibrillary tangles (NFTs) formed by the accumulation of insoluble and hyperphosphorylated tau. The 40-42 amino-acid β-amyloid peptide is the major component of the amyloid deposits. It is produced from a larger protein, the amyloid precursor protein, by proteolytic cleavage. Tau is a soluble microtubule-binding protein, which stabilizes the microtubules in axons. Hyperphosphorylation of tau protein causes destabilization of microtubules and subsequent dissociation of tau, which in turn aggregates to form NFTs.
Glycogen synthase kinase-3 (GSK-3) is a serine/threonine protein kinase that participates in a plethora of cellular processes, e.g. cell proliferation, microtubule dynamics and gene transcription. Several studies have linked GSK-3 to the primary abnormalities associated with AD; particularly the phosphorylation of tau. Two related isoforms of GSK-3 exist in mammals, GSK-3α and β, which share 98% homology in their catalytic domains and have similar biochemical properties. Considering the homology of GSK-3α and β within the ATP-binding pocket it appears difficult to identify an inhibitor that differentiates the two isoforms. All GSK-3 inhibitors developed until now are able to inhibit the two isoforms with almost similar potency. A plethora of GSK-3 inhibitors has been described and most of the effects were observed in vitro and cellular studies. These studies and the ongoing patent filing indicate that GSK-3 is a potential drug target not just for the treatment of AD. Inhibition of GSK-3 by selective inhibitors with excellent pharmacokinetic properties and blood-brain barrier permeation holds high potential for the treatment of AD. Nevertheless, there is no clinically approved GSK-3 inhibitor.
Based on a simplified scheme of known and important interactions of GSK-3 inhibitors with the ATP binding pocket we generated hypotheses for improved interaction of with this site. An appropriate decoration resulted in a more extended occupation of the ATP binding site. The most potent inhibitors displayed IC50 values in the low nanomolar range and good kinase selectivity. Noteworthy, several compounds showed the highest GSK-3α selectivity reported so far. The contribution of GSK-3α and GSK-3β to the pathology of Alzheimer’s disease is still subject of an ongoing debate. Thus, the identification of potent GSK-3α-selective inhibitors, endowed with favorable pharmacokinetic properties, may elucidate the effect of GSK-3α inhibition in AD models. Therefore, these compounds may be useful tools and starting points for the synthesis of GSK-3α selective inhibitors with enhanced pharmacokinetic properties. The GSK-3 inhibitors were further profiled for efficacy and toxicity in the wild-type (wt) zebrafish embryo assay, which is an established model system for the validation of GSK-3 inhibitors.